The present invention can be applied in digital radio systems in which subscriber equipments send a specific call set-up message when they want to establish a radio connection with a base station and in which the call set-up message comprises a training sequence. Typically, a call set-up message is sent on a random access channel, which means that message transmission does not have an exact predetermined starting and ending moment. It is typical of a random access channel that transmissions on the channel may coincide and so a transmission may have to be repeated. The base station does not know when the terminal equipments need to communicate, and so it cannot allocate them turns to transmit beforehand.
Detection of a call set-up message at a base station is demanding, since the base station does not know beforehand the moment when a signal will arrive, the power level of the signal, or its possible frequency deviation from the nominal frequency of the call set-up channel. To facilitate detection of a message, the message usually contains a number of previously known bits, a sequence of previously known bits being called a training sequence. A training sequence is commonly used in digital radio systems, since it can be used in computing the properties of the channel, such as an impulse response, by which one can attempt to correct any distortions that the channel may cause in a signal.
In prior art solutions, a decision whether or not a signal received in a given time slot comprises a call set-up message is made after an equalizer when an decoded assumed training sequence is compared with a known training sequence. If there are many error bits, the signal has probably contained noise rather than a message. Call set-up should be rendered as quick and reliable as possible, since this adds to user comfort. The quicker a call set-up message can be detected, the quicker a call can be set up between a base station and a terminal equipment.